Teleoperated or computer assisted medical systems often employ a master control that a physician or other medical personnel can use to control actuated slave medical instruments. A medical instrument may, for example, include a tool such as a scalpel, forceps, or a cauterizing tool, and a surgeon may operate a master control similar to a joystick to provide control signals to a control system. The control system can then convert the control signals into actuation signals that drive actuators to move the instrument, for example, to cut, clamp, or cauterize a patient's tissue so that the tool movement follows the master control movement. One potential concern for such systems is inadvertent or uncontrolled movement of the master control, because a patient could be injured if uncontrolled movement of the master control causes uncontrolled operation of the tool that interacts with a patient's tissue. Surgeons can be trained to avoid situations where uncontrolled movement is possible, but additional techniques or fail safes may be desirable to prevent uncontrolled movement.
One way to reduce the chance of uncontrolled movement of the master control is to restrict movement of the mechanical components of the master control. For example, a master control may be balanced or actively driven so that gravity does not cause the master control to drift away from any position in which a physician may leave the master control. Other ways to reduce the chance of uncontrolled movement of an instrument may use a “locked” mode that decouples the master control from the instrument, so that in the locked mode, movement of the master control does not cause corresponding movement of the instrument. The locked mode might be automatically activated when a physician is not in the proper position for use of the master control. In particular, the system may default to the locked mode unless sensors detect that a physician is in the proper position for use of the system including, for example, being in position to view any movement of an end effector of the medical instrument. However, after a physician has taken a medical system out of the locked mode, the physician might release a master control while remaining in position to use the medical instrument. Releasing the master control under such circumstances may create the risk of the master control moving without physician input, resulting in uncontrolled motion of the medical instrument. For example, gravity compensation in the master control may be imperfect, or a physician's knee or hand may accidentally bump the master control while the physician is in position to use the instrument and the instrument is not in the locked mode.